This invention relates to a method of doping polysilicon layers on a silicon wafer. In the production of semiconductor chips, polysilicon layers are doped with phosphorous to form conductive layers.
One way to dope the polysilicon with phosphorous uses a two-step doping process. In the first step, called the predeposition phase, a phosphorous glass layer is formed on top of the polysilicon layer on a wafer. In the second step, called the drive-in-diffusion phase, phosphorous diffuses into the silicon from the phosphorous glass. The two-step doping process allows for lower temperatures during the doping than a pure gaseous doping, because more of the potential doped material is at the surface of the polysilicon layer.
FIG. 1 is a cross-sectional view of the semiconductor wafer with a silicon substrate 2, a doped silicon region 4, a gate oxide 6, and a field oxide 8. An undoped polysilicon layer 10 is formed on top of the wafer.
The wafer is then placed into an oven which is filled with nitrogen. The temperature of the oven is increased to the temperature required for the desired rate of the drive-in diffusion of phosphorous from the phosphorous glass layer. The temperature of the oven also determines the solid solubility of the phosphorous into the polysilicon. The oven is then stabilized at that temperature. Next, in the predeposition phase, oxygen gas and phosphorous oxychloride gas (POCl.sub.3) are then introduced into the oven. The POCl.sub.3 gas reacts with the oxygen in the oven to form a phosphorous glass on the polysilicon layer 10. The reaction of the oxygen gas and phosphorous oxychloride gas has the equation EQU 4POCl.sub.3 +3O.sub.2 .fwdarw.2P.sub.2 O.sub.5 +6Cl.sub.2
The phosphorous glass essentially has the formula P.sub.2 O.sub.5 and the phosphorous glass forms a layer 12 shown in FIG. 2 on top of the semiconductor wafer.
The drive-in diffusion phase occurs when, in the heat of the oven, the phosphorous glass layer 12 on top of the polysilicon layer 10 reacts with the polysilicon following the equation EQU 2P.sub.2 O.sub.5 +5Si.fwdarw.4P+5SiO.sub.2
In this reaction, the phosphorous diffuses into the polysilicon layer 10 from the phosphorous glass layer 12. This phosphorous makes the polysilicon layer more conductive. After a period of time, the temperature of the oven is cooled down and the semiconductor wafer is removed from the oven. Thereafter, in a deglazing step, the phosphorous glass is removed from on top of the polysilicon layer 10.
FIG. 3 shows a doped polysilicon layer 14 covering the silicon wafer. This doped polysilicon layer can then be shaped as desired to connect structures on the silicon wafer.
Although a conductive polysilicon layer can be formed with the prior art method, it is desired to improve the POCl.sub.3 doping process.